1. Field of the Invention
The invention is generally related to the installation of underwater structural components for offshore production systems and more particularly to the installation of such components in deep water.
2. General Background
The installation of underwater structural components for offshore production systems, such as templates, piles, conductors, and tower bottom sections, has been accomplished using derrick barge cranes. The template is a tubular steel framework that serves as a piling and conductor guide and equipment platform that extends from the sea floor up to approximately thirty feet above the mudline. The pilings consist of large steel tubes which secure the template or the tower bottom section to the sea floor and penetrate the soil to as much as six hundred feet. Conductors are steel pipes which are driven into the soil through guides connected to the template that are used for drilling wells. The tower bottom section is the lower portion of a large offshore platform.
The most common means of installing underwater templates, piles, and conductors is the use of cranes mounted on derrick barges. Cranes have capacity limitations for underwater blocks and most derrick barge cranes are not equipped with underwater blocks. An advantage of the use of a crane is that it does not require a load transfer from the crane to another system. A disadvantage of using a crane is the restriction of the offshore underwater installation to relatively light loads and shallow water due to the capacity limitation of underwater blocks. Another disadvantage of using a crane is that it ties up the crane from doing other work during lowering operations and limits the capability of the crane to provide assistance during emergencies that may arise.
A second installation means is a pulling system mechanism with grippers and a traveling block sheave assembly. A general advantage of this system is that it can handle a heavier load than a crane, depending upon the size of the pulling system and the pulling line. A disadvantage of this system is that it requires load transfer from the crane to the pulling system and sheave system. Another disadvantage is that it ties up the crane from doing other work during lowering operations and restricts the cranes from the capability of any assistance during emergencies.
A third system is a multiple hoist system that provides the advantages of freeing the barge crane to do other work and increases the offshore underwater installation to heavier loads and in deeper water. Disadvantages include: Load transfer is required from the crane to the multiple hoist system. One operator is required for each winch. The hoists are operated at different and low speeds. There is no master control console to synchronize all hoist drums. There is no equalizer beam to control the differential load and elongation between the wire ropes and the variable hoist speed. Separate offshore lifts are required for each component of the system and the system requires offshore assembly.
It can be seen that the current state of the art does not adequately address the needs encountered in lowering components for offshore production systems in deep water.